CHAPTER VII 
EVIDENCES FROM GEOGRAPHIC DISTRIBUTION 
[Just as palaeontology may be said to be a study of the vertical 
distribution (distribution in time) of organisms, so geographic distribu- 
tion may be called a study of the horizontal distribution of organisms, 
on the earth’s surface at any given time (spatial distribution). Weare 
chiefly to be concerned with the present spatial distribution of animal 
and plant species, but equally interesting studies have been and still 
may be made of the horizontal or contemporaneous existence of 
extinct forms. Much new knowledge has been gained by combining 
the data of palaeontology with those of geographic distribution. In 
fact, neither field can be studied profitably without recourse to the 
other. This fact.was clearly perceived by J. A. Thomson in his little 
manual on Evolution when he combined the two types of evidence in 
one chapter under the title “Evidences of Evolution from Explorer 
and Palaeontologist.” 
It was a consideration of the present and of the past distribution 
of Edentates that led Charles Darwin to his first clear concept of 
descent with modification. In his voyage on the “Beagle” he found 
that present-day Edentates (armadillos, sloths, anteaters), a very 
peculiar group of archaic mammals, are practically confined to South 
America. When he also found that the only fossil Edentates, resem- 
bling but also differing from the existing types, are also confined to 
South America, he easily arrived at the only inference permitted by 
the facts: that the present Edentates are the modified descendants 
of the Edentates of the past. 
The following quotations from both an older and a recent writer 
will give the reader a clear idea of the ways in which the general facts 
of geographic distribution bear witness to the truth of the evolutionary 
principle.—Ep.] 
“The theory,” says Wallace,‘ “which we may now take as estab- 
lished—that all the existing forms of life have been derived from other 
forms by a natural process of descent with modification, and that this 
same process has been in action during past geological time—should 
1 From A. R. Wallace, Darwinism (1889). Used by special permission of the 
publishers, The Macmillan Company. 
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